The invention relates to the field of sensors for the direct measurement of electromagnetic power, and to their manufacture.
It is very useful in many industrial fields to measure the power of an electromagnetic wave.
To know the power levels at all stages is crucial for the performance of most microwave equipment, whether during the design phase of this equipment or during their normal use. Each component must receive a suitable power level from the upstream component and must itself deliver the correct level to the downstream component. If the power level is too low, the useful signal is drowned in noise. If the level is too high, distortion and saturation problems arise. Moreover, at microwave frequencies, the power is, for metrologists, a well-defined characteristic, unlike the voltage or the current.
The power of an electromagnetic wave is generally measured by means of isolated electromagnetic power sensors or radiofrequency (RF) and ultrahigh frequency (UHF) watt meters comprising such sensors.
Electromagnetic power sensors using thermal diodes or sensors, such as bolometers or thermocouples, for measuring the power are already known.
In diode sensors, the diode is mounted in a detection cell and the average voltage at the output of this cell is used to measure the power upstream of the diode. The advantages of such power sensors are, for example, the sensitivity (powers as low as xe2x88x9270 dBm, i.e. 0.1 nW, may be measured), the dynamic range (typically 50 dB), the ability to be integrated into silicon planar circuits and the speed (overall response time approximately 1 xcexcs). However, these sensors are nonlinear for power levels above xe2x88x9220 dBm (i.e. 10 xcexcW). To be able to measure higher powers, an attenuator has to be placed in the measurement chain, but this reduces the accuracy of the measurement. In this case, manufacturers are obliged to provide diode sensors having a sophisticated electronic circuit comprising a microprocessor and memories, so as to correct these nonlinearities. This increases the cost of such sensors.
In thermal sensors, two types of measurement are mainly used, namely xe2x80x9ccompensatedxe2x80x9d measurement and xe2x80x9cdirectxe2x80x9d measurement.
In the case of a compensated measurement, the load absorbs the electromagnetic power, but also that delivered by an additional DC electric current, and is maintained at a constant temperature by regulating the level of the power absorbed (any variation in the electromagnetic power is compensated for by a variation in the opposite direction of the power delivered by a DC electric current). Measuring the DC electric current gives the electromagnetic power level of the wave. However, the manufacture of this type of sensor has the drawback of being technologically complex.
In the case of a direct measurement, the electromagnetic wave is absorbed in a matched load and converted into heat (the principle of bolometry); the heat-up of the structure is then measured with a thermistor or a thermocouple. This type of sensor comprises two distinct elements: the matched load, which absorbs the power and converts it into heat, and the thermometer, which may be a thermistor or a thermocouple. The advantages of such thermal sensors are the measurement accuracy, the bandwidth and the fact that high-power measurements are possible.
One objective of the invention is to provide a sensor for the direct measurement of electromagnetic power, giving an accurate measurement, having a relatively wide bandwidth, and allowing measurements at high power, while still having a low manufacturing cost.
This objective is achieved, according to the invention, by virtue of a sensor for the direct measurement of electromagnetic power, comprising a guiding structure for injecting the power, a dissipative load and a thermometer, characterized in that a single element forms the load and the thermometer.
For this purpose, such a sensor is a thermal sensor. It therefore has the advantages of this type of sensor, while still having, by virtue of the fact that the load and the thermometer constitute a single element, a simplified technological construction and consequently a lower manufacturing cost.
Advantageously, the guiding structure is a planar structure, especially in the form of a microstrip line. The sensor can then be integrated into a structure which is itself planar.
Also advantageously, the transition between the guiding structure and the load is made with a pointed profile. This transition may be produced with a load having a pointed profile and/or a guiding structure having a pointed profile. Such a technique is also called tapering. This technique makes it possible to have, as input for the load, a characteristic impedance close to the 50 ohm nomical impedance of the line. The tapered line absorbs the incident microwave power progressively, while minimizing therefore the parasitic reflections. The power is dissipated in the form of heat, and hence the temperature of the line increases and, as a consequence, the DC resistance of the line varies (without this affecting the ultrahigh frequency behavior of the line). All that is required therefore is to measure this variation.
The term xe2x80x9cloadxe2x80x9d used in this text corresponds to a localized load and/or a distributed load.
According to another aspect, the invention is a process for manufacturing sensors for the direct measurement of electromagnetic power, comprising the production of a guiding structure for injecting the power, of a dissipative load and of a thermometer, characterized in that the load and the thermometer are produced in the form of a single resistive element.
Advantageously then, according to this process, the transition between the guiding structure and the load is made with a pointed profile.
According to another aspect, the invention is a device comprising a sensor such as that presented above.
Advantageously then, the latter comprises two elements, of which:
one is a sensor for the direct measurement of electromagnetic power, which comprises a guiding structure for injecting the power, a dissipative load and a thermometer, the latter two components forming a single element; and
the other serves for regulating the temperature of the sensor for the direct measurement of electromagnetic power.
This makes it possible to compensate the variations in room temperature. According to one version, the device serving for regulating the temperature perhaps a Peltier-effect device. According to another version of the device according to the invention, this comprises two elements identical to said sensor, but only one of them is exposed to the electromagnetic wave whose power it is desired to measure.
A differential measurement is then made, in which both sensors are placed in a Wheatstone bridge, and one of them is exposed to the electromagnetic wave while the other is not. This guarantees reproducible measurement of the power whatever the temperature drift of the sensor.